Re: I Never Thought Of This!!!

Posted by Dave on September 18, 2000 at 09:07:23 from (128.113.83.150):

In Reply to: I Never Thought Of This!!! posted by Bill {Antique Acres} on September 16, 2000 at 16:08:12:

I'm almost afraid to jump into this one, but Bill actually is right on the generator theorey. I don't want to start a physics class here, but the voltage a generator will develop is determined by the rpm, the number of windings, and the magnetic field strength. The power output is not constant: the rated wattage is a maximum power output, not the stated power output. If you run a 12 v generator at no load (0 amps or open circuit), it will produce 12 volts, and 12x0=0 watts. If you run the same gen under a 1 amp load, then it produces 12x1=12 watts, etc. Note that this amperage flows through the gen windings, which have their own internal resistance, and heat up as amperage increases. Eventually the winding amperage and heat is so great that the windings start to break down. This is what limits generator output.

It is true on these self-excited generators that the field current is produced by the generator, and the magnetic field strength could be increased by increasing field current. The problem here is that increasing the current flowing in the field windings produces more heat which will break down the windings. Since field strength can't safely be increased (much), this leaves either increasing rpm's or increasing the number of windings.

That's the theorey. In practice, generators (and motors, too) have what's called a "speed droop:" they produce less voltage under load than when under no load. This is due to voltage drop inside the windings themselves due to internal resistance. Say the windings have a resistance of 0.1 ohm. Run this on a circuit with a 0.9 ohm resistance. The 12 volt generator will still put out 12 volts, although you'll "see" only 10.8 volts at the terminals because you lost 1.2 volts inside the windings (12volts/1 ohm total = 12 amps current. 12 amps x 0.1 ohm = 1.2 volts lost inside the gen).

For this reason, the no-load voltage of small generators is higher than stated: my 6-volt gen puts out 8.4 volts no-load with the tractor at idle, for instance. If I put it under load, voltage drops, because of the voltage "lost" inside the gen.

As far as tractors running fine with 6 volt gens on 12-volt systems: I guess you could increase field current to increase the voltage output: if the windings are heavy enough to take the extra current. You could also put a little smaller pulley on to the up the rpm's. Also remember that just as a 6-volt gen puts out more than 6 volts at idle, then at higher engine speeds they will produce a higher voltage yet. So if it breaks the 12-volt mark at higher engine speeds, I see no reason why it won't charge. The max amperage available will be less due to the size of the windings, but with a good battery you may not even notice. Hope that helps.
Dave

Follow Ups:

• Re: Re: I Never Thought Of This!!! T_Bone 10:22:06 09/18/00 (2)
  • Re: Re: Re: I Never Thought Of This!!! Dave 11:45:17 09/19/00 (1)
    • Re: Re: Re: Re: I Never Thought Of This!!! T_Bone 09:35:43 09/20/00 (0)

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